The proposed research deals with several aspects of neurotransmission/neuromodulation in the spinal cord and dorsal column nuclei of rats and cats. One of the three main goals is to establish whether glutamate immunoreactivity co-exists with immunoreactivity for substance P or other peptides in the same terminal of primary afferent fibers to the superficial laminae of the spinal dorsal horn. For this purpose, immunocytochemical double-labeling methods for electron microscopy will be employed. Experiments will be also performed to establish co-existence of glutamate with peptides in neuronal somata of dorsal root ganglia. Light microscopical double-labeling methods of the type already employed in this laboratory to demonstrate coexistence of glutamate and substance P will be used for this part of the proposed research. A second series of experiments is designed to establish whether at least some of the spinal neurons at the origin of non-primary (postsynaptic) afferents to the dorsal column nuclei and at the origin of the spino-thalamic tract in rats contain substance P. The main strategy for this part of the proposed research consists of double-labeling spinal neurons by retrograde transport of Diamidino Yellow injected in either dorsal column nuclei or n. ventralis posterolateralis of thalamus by immunocytochemistry for substance P. The retrograde double-labeling experiments will be supplemented by observations with an anterograde double-labeling technique and by an investigation of synaptic relation of substance P-positive terminals in both the dorsal column nuclei and the n. ventralis posterolateralis of thalamus. A third goal is to investigate the significance of inhibitory GABAergic mechanisms in the dorsal column nuclei of cats. The experiments consist of studying the electrophysiological responses of thalamic projecting neurons in the cuneate nucleus to various types of peripheral stimuli during iontophoretic ejection of bicuculline methobromide, known to block GABAA receptors. With these experiments it is expected to gather information concerning the role of inhibition in the modulation of sensory input to neurons at the first level of the medial lemniscal system.